This invention relates to lacquer binders and provides new alkyd resins and alkyd resin mixtures for use in stoving water lacquers.
In recent years, numerous efforts have been made to provide stoving lacquers which, on stoving, give off hardly any volatile atmosphere-polluting substances. For example, it has frequently been proposed to use as binders carboxyl group-containing alkyd resins which can be dissolved in aqueous solvent mixtures after neutralization with amines. Alkyd resins suitable for this purpose generally have acid numbers of from 40 to 80 mg of KOH/g. The use of alkyd resin dispersions which have acid numbers of from &lt;30 mg of KOH/g or even &lt;10 mg of KOH/g, but which contain emulsifiers, is also known.
Finally, alkyd resin microemulsions are known which have acid numbers of from 5 to 35, but which contain no volatile solvents and only emulsifiers of the type which lose their hydrophilicity on stoving. Thus, U.S. Pat. No. 4,271,051 describes alkyd resins containing numerous 2,3-dihydroxy propyl groups through the reaction of free carboxyl groups with glycidol. By selecting suitable amines, emulsifiers and crosslinking agents (generally melamine resins), it is possible to produce from these alkyd resins optiocally clear aqueous preparations (microemulsions) which on stoving give off hardly any harmful constituents to the atmosphere.
Although these alkyd resins are largely satisfactory both in terms of processibility into water lacquers and in terms of environmental considerations, there is still a need to improve the technical properties of lacquer films obtainable therefrom. Thus, the resistance to water (as determined by the "sweatbox" test) and also the flexibility of the lacquer films, despite their considerable hardness, are in need of improvement without however producing any adverse effects upon their other favorable technical properties.
It is known from U.S. Pat. No. 4,039,495 that trimethylol nitromethane may be used as an additive for increasing the flexibility of alkyd resin lacquers. Unfortunately, this highly hydrophilic additive reduces the resistance of the lacquer films to water very considerably. Attempts to obtain flexible lacquers by increasing the fatty acid content of the alkyd resins were also unsuccessful because resins of this type could not be made up into water lacquers without the use of solvents.